Merge pull request immersive-web#100 from kvark/doc-submission

Add a small document about the submission model

design/CommandSubmission.md

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+# Command Submission
+
+Command buffers carry sequences of user commands on the CPU side.
+They can be recorded independently of the work done on GPU, or each other.
+They go through the following stages:
+
+creation -> "recording" -> "ready" -> "executing" -> done
+
+Command buffers are created from and submitted to a command queue.
+Creation and submission do not have to follow the same order.
+The queue is also used to signal fences, allowing the user to know when the command buffers are done.
+
+## Detailed Model
+
+Users issue rendering and compute commands (such as resource bindings, draw calls, etc) via command buffers.
+The concept of `WebGPUCommandBuffer` matches the native graphics APIs.
+Those command buffers go through the following stages in their life cycle.
+It starts with creating a new `WebGPUCommandBuffer` from a `WebGPUCommandQueue` instance.
+From this point, the command buffer is considered to be in "recording" state.
+
+Commands can be encoded independent of anything done on `WebGPUDevice` or the underlying GPU.
+The recording is CPU-only operation, and multiple command buffers can be recorded independently on web workers.
+(TODO: disallow recording multiple command buffers on the same thread/web worker?).
+Recording usually consists of a number of passes, be it render or compute, with occasional copy operations inserted between them.
+
+Since a programmable pass defines the resource binding scope, synchronization rules, fixes the resource usage, and exposes a number of specific operations, we encapsulate the encoder of a pass into a separate object, such as `WebGPURenderPassEncoder` and `WebGPUComputePassEncoder`.
+The pass encoder object can be obtained from a command buffer by calling `beginRenderPass` or `beginComputePass` correspondingly.
+The command buffer is expected to be in "recording" state, or otherwise a synchronous error is triggered.
+No operations may be done on the `WebGPUCommandBuffer` if there is an open pass being encoded to it.
+Calling any methods on the command buffer with an open pass, or submitting it to the command queue, triggers a synchronous error.
+A pass encoding consists of state setting code and draw/dispatch calls, which are all methods on the corresponding encoder object.
+In order to close a pass, the user calls `WebGPUProgrammablePassEncoder::endPass`, which returns the owner `WebGPUCommandBuffer` object.
+Passes cannot straddle command buffers, and a command buffer may contain multiple passes.
+
+In order to finish recording a command buffer, the user calls `WebGPUCommandBuffer::finish` method, which transitions it from "recording" to the "ready" state.
+It is valid to transfer this object between web workers.
+When "ready", a command buffer can only be submitted for execution via `WebGPUCommandQueue::submit`, and no recording operations are available.
+This method gets a sequence of command buffers and submits them (in the given order) to the GPU driver.
+There are a few hidden (from the user point of view) stages here before the command buffer actually reaches the GPU.
+
+Once submitted, the command buffer switches to "executing" state, which means the command buffer will execute (both on the CPU and GPU) in finite time.
+If the WebGPU implementation fails to submit the command buffer due to a problem with recorded content (e.g. exceeding the limit for the instance count in a draw call), it is turned into an internally null object, and the asynchronous error is reported.
+The feature to re-use command buffers for multiple submissions is still being discussed, and until this is clear, we consider the `WebGPUCommandBuffer` to be moved into submission.
+Any operations on a command buffer in the "executing" state, other than dropping it (which is what the user is expected to do), would trigger a synchronous error.
+
+If the submission is successful, then at some point in time the GPU will be done processing it.
+The WebGPU implementation takes the responsibility to detect this moment and gracefully recycle/destroy this command buffer, when it's safe to do so.